This application is based upon claims the benefit of priority from the prior Japanese Patent Application No. 11-153743 filed Jun. 1, 1999 the entire contents of which are incorporated herein by reference.
The present invention relates to an optical displacement sensor, and in particular, to an optical displacement sensor for detecting displacement of a precision mechanism.
First, as a conventional technique for optical displacement sensors of the above kind, the optical displacement sensor disclosed in Jpn. Pat. Application No. 11-6411 and comprising a vertical cavity surface emitting laser as a light source will be described.
The configuration and operation of the optical displacement sensor according to this conventional technique will be explained with reference to FIGS. 9A and 9B.
The optical displacement sensor according to this conventional technique is configured so that a transmissive scale 2 having a periodic pattern formed thereon is irradiated with laser beams emitted from a semiconductor laser that is a coherent light source 1 so that a particular portion of the diffraction interference pattern generated by the irradiation is detected by a photodetector 3.
The sensor operation of this type will be described below.
First, as shown in FIGS. 9A and 9B, each configuration parameter is defined as:
z1: distance between the light source and a surface of the scale on which a diffraction grating is formed,
z2: distance between the of the scale with the diffraction grating formed thereon and a light receiving surface of the photodetector,
p1: pitch of the diffraction grating on the scale,
p2: pitch of the diffraction interference pattern on the light receiving surface of the photodetector,
xcex8x: spread angle of light beams from the light source with respect to a pitch direction of the diffraction grating on the scale, and
xcex8y: spread angle of light beams emitted from the light source, in a direction perpendicular to the xcex8x (the spread angle of light beams refers to the angle between a pair of boundary lines 6 at each of which the light beam intensity is half of a peak value).
The xe2x80x9cpitch of the diffraction grating on the scalexe2x80x9d means the spatial cycle of the periodic pattern formed on the scale 2 and having its optical characteristics modulated.
In addition, the xe2x80x9cpitch of the diffraction interference pattern on the light receiving surface of the photodetectorxe2x80x9d means the spatial cycle of the intensity distribution of the diffraction interference pattern generated on the light receiving surface of the photodetector 3.
According to the light diffraction theory, an intensity pattern similar to the grating pattern on the scale 2 is generated on the light receiving surface of the photodetector 3 when the z1 and z2 defined above have such a particular relationship as meets the relationship shown in the following Equation (1):
(1/z1)+(1/z2)=xcex/kp12xe2x80x83xe2x80x83(1)
where xcex denotes the wavelength of light beams emitted from the light source and k is an integer.
In this case, other configuration parameters can be used to express the pitch p2 of the grating pattern on the light receiving surface as shown in the following Equation (2):
p2=p1(z1+z2)/z1xe2x80x83xe2x80x83(2)
When the scale 2 is displaced in the pitch direction of the diffraction grating with respect to the light source 1, the intensity distribution of the grating pattern moves in the displacement direction of the scale 2 with the same spatial cycle maintained.
Thus, by setting the spatial cycle p20 of light receiving areas 4 of the photodetector 3 to be equal to the value p2, a periodical intensity signal is obtained from the photodetector each time the scale 2 moves in the pitch direction by the pi, thereby allowing detection of the displacement of the scale 2 in the pitch direction.
The operation of conventional displacement sensors will be explained below.
Laser beams emitted from the vertical cavity surface emitting laser that is the coherent light source 1 are formed by the diffraction grating on the scale into a diffraction interference pattern on the light receiving surface of the photodetector 3, the pattern having a constant cycle pi (z1+z2)/z1.
Since the light receiving areas 4 on the photodetector 3, constituting light intensity-detecting means, are formed in the pitch direction of the diffraction grating at distances of np1 (z1+z2)/z1, these light-receiving areas detect only the same particular phase portion of the diffraction interference pattern on the light receiving surface.
When the scale 2 is displaced in the pitch direction of the diffraction grating by x1, the diffraction interference pattern on the light receiving surface is displaced in the same direction by x2=x1 (z1+z2)/z1. Consequently, each time the scale 2 is displaced in the pitch direction of the diffraction grating by one pitch, the light intensity-detecting means provides output signals with a periodically varying intensity.
A primary axis of light beams emitted from this surface emitting laser light source is shown at reference numeral 5, and beam boundaries at which the light beam intensity is half that of the primary axis are shown at reference numeral 6.
Additionally, a remote tangent to each of the beam boundary curves 6 is shown at reference numeral 6xe2x80x2, and the angle between the tangents 6xe2x80x2, which are opposed to each other with respect to the primary axis of the light beams, is defined as xcex8x and xcex8y in directions x and y, respectively, and the xcex8x and xcex8y are referred to as the spread angle of the light beams.
In the surface emitting laser, by freely setting the dimensions of an emission window in an element to vary beam diameters xcfx89ox, xcfx89oy on an emission surface, the xcex8x and xcex8y can be set at ones of a broad range of values due to diffraction of the light beams.
Further, when an inclined base 11 is provided, the grating surface of the scale 2 and the light receiving surface of the photodetector 3 are inclined with respect to the primary axis of the light beams emitted from the laser light source. Accordingly, when light emitted from the laser light source 1 is reflected from a surface of the scale 2 or the photodetector 3, it can be prevented from returning to the laser light source 1 to restrain optical reflection noise in the laser light from being superposed on output signals from the sensor.
Thus, the optical displacement sensor using the vertical cavity surface emitting laser light source shown in FIGS. 9A and 9B can sense the displacement of the scale more accurately and reliably.
On the other hand, the principle on which the displacement sensor is based when the diffraction grating is irradiated with parallel beams will be described with reference to FIG. 10.
A measuring movable grating 103 is a rectangular-wave grating comprising transparent and opaque portions alternated at equal distances and mounted on a measured object in its moving direction.
On the other hand, a small fixed grating 104 having the same shape as the movable grating 103 is located opposite and close to the movable grating 103, and in this arrangement, a light source 101 applies parallel beams via a lens 102 so that transmitted beams are detected by a photodetector 106 via a lens 105.
When fringe directions of the two gratings 103, 104 are maintained exactly in parallel, the scale distance between the gratings is defined as z, the scale pitch is defined as p1, the oscillation wavelength of a laser is defined as xcex, and k denotes an integer. Then, when these values are adjusted to meet the following equation:
z=kp12/xcexxe2x80x83xe2x80x83(3)
a luminous flux entering the photodetector 106 repeatedly increases or decreases each time the measuring movable grating 103 moves by one pitch.
In this case, if the two gratings are closely contacted with each other, the modulation of-optical outputs will be 100%. Due to the presence of the movable portion, however, a certain gap is required between these gratings.
Of the above described conventional techniques, the optical displacement sensor shown in FIGS. 9A and 9B includes the inclined base 11 to incline the primary axis of light beams from the surface emitting laser light source 1 through the angle xcfx86, in order to prevent reflected light. The assembly of this sensor, however, is difficult.
Thus, it is a first object of the present invention to realize an optical displacement sensor that accepts easy planar assembly without the need to incline the light source.
Additionally, the conventional displacement sensor using parallel light beams uses the lenses 102, 105 that allow parallel beams to enter the scale so that transmitted beams focus on the photodetector, as shown in FIG. 10. Consequently, the optical axes of these lenses must be adjusted, and it is difficult to integrate them into the sensor through a semiconductor process.
In addition, the conventional displacement sensor using the vertical cavity surface emitting laser as the light source uses light penetrating a substrate in order to directly monitor the power of the light source. As a result, the oscillation wavelength is limited.
Further, the conventional displacement sensor using the vertical cavity surface emitting laser as the light source requires the inclined base 11 to be produced accurately, thereby increasing costs.
Therefore, it is a second object of the present invention to realize an inexpensive high-performance optical displacement sensor that accepts easy planar assembly and that can be integrated through a semiconductor process.
It is an object of the present invention to achieve the above described first and second objects to provide an inexpensive optical displacement sensor that can be assembled easily and that can sense displacement of a scale accurately and reliably.
(1) To attain the above objective, according to an aspect of the present invention, there is provided an optical displacement sensor comprising:
a laser light source which emits light beams having a predetermined beam shape;
a scale that is displaced in a fashion traversing the light beams emitted from the laser light source and that includes a periodic pattern formed thereon to generate a diffractive interference pattern from the light beams;
a photodetector which receives at least a portion of the diffractive interference pattern; and
an optical element which refracts the light beams emitted from the laser light source to the scale;
wherein:
the photodetector includes a photodetector array comprising plural light receiving areas having a period of np1 (z1+z2)/z1 in a pitch direction of the diffractive interference pattern on a surface of the photodetector,
a distance between a light beam emission surface of the laser light source and a surface of the scale is defined as z1,
a distance between the surface of the scale and a light receiving surface of the photodetector is defined as z2,
a pitch of the periodic pattern on the scale is defined as p1, and
n denotes a natural number.
Embodiments 1 to 5, described later, correspond to this aspect of the present invention.
In the above configuration, the xe2x80x9cperiodic patternxe2x80x9d may be a diffraction grating having formed therein a periodic/modulation pattern for an optical characteristic such as amplitude or phase and may include any diffraction grating such as a reflection or transmission grating which generates the diffractive interference pattern on the light receiving surface.
Laser beams emitted from the laser light source with the optical element integrated therewith are formed by the periodic pattern on the scale into the diffractive interference pattern on the light receiving surface, the pattern having a constant cycle of p1 (z1+z2)/z1.
Since the light receiving areas on the photodetector, constituting light intensity-detecting means, are formed at distances of np1 (z1+z2)/z1, these light-receiving areas detect only the same particular phase portion of the diffractive interference pattern on the light receiving surface.
When the scale is displaced in the pitch direction by x1, the diffractive interference pattern on the light receiving surface is displaced in the same direction by x2=x1 (z1+z2)/z1. Consequently, each time the scale is displaced in the pitch direction by one pitch, the light intensity-detecting means provides output signals with a periodically varying intensity.
Light beams emitted from the laser light source having the optical element integrated therewith for bending the emission direction through a predetermined angle are inclined through a predetermined angle xcfx86 with respect to a normal to an element surface. Consequently, when light emitted from the laser light source is reflected from a surface of the scale or the photodetector, the reflected beams are prevented from returning to the laser to restrain optical reflection noise in the laser light from being superpose on output signals from the sensor.
(2) To attain the above objective, according to another aspect of the present invention, there is provided an optical displacement sensor according to (1) characterized in that the laser light source is of an edge emitting type, and in that:
the optical element is a reflection surface formed on a semiconductor substrate.
Embodiment 3, described later, corresponds to this aspect of the present invention.
A major-axis direction of beams from the edge emitting laser corresponds to the spread of the beams in a vertical direction relative to a laser junction surface on its end surface.
This beam divergence can be set at 20xc2x0 or smaller by setting waveguide thickness d at 0.05 xcexcm or smaller and xcex94n at 0.2 or smaller.
In addition, in a minor-axis direction, the beams spread in a horizontal direction relative to the junction surface over a predetermined stripe width.
By setting the spread beam divergence angle in the minor-axis direction at 10xc2x0 or smaller, a stable single mode beam is obtained in the horizontal direction and the output of the edge emitting laser can be increased to restrain a decrease in the S/N ratio of photodetector signals.
Dry etching is used to form the reflection surface on part of a stripe waveguide in the edge emitting laser as an external mirror inclined through a predetermined beam angle so that laser beams emitted from end surfaces formed by means of dry etching are reflected from the reflection surface acting as the external mirror and then applied to the surface of the scale 2.
Additionally, a monitor element having the same structure as the laser section is provided on one of the end surfaces to monitor optical outputs from the edge emitting laser.
(3) To achieve the above objective, according to a further aspect of the present invention, there is provided an optical displacement sensor according to claim (1) wherein the optical element is a grating formed in a waveguide.
Embodiments 1 and 2, described later, correspond to this aspect of the present invention.
As shown in FIG. 2, when a beam emission angle from a normal direction of a surface of a grating 50 is defined as xcfx86, the cycle of a grating in a waveguide is defined as xcex9, the waveguide-equivalent refractive index is defined as N, and the laser oscillation wavelength is defined as xcex, and if the grating is formed in such a manner as to meet the following equation:
sin xcfx86+xcex/xcex9=Nxe2x80x83xe2x80x83(4)
then, parallel beams inclined through an angle xcfx86 are emitted.
When the pitch of the grating 50 is constant, beams are applied to the scale 2 at the predetermined beam angle xcfx86.
Then, when the stripe width of the waveguide is set in a manner such that the beam divergence is single modal, the beam divergence angle is about 10xc2x0, so that a beam spot pattern 15 is defined on the scale surface.
This pattern does not spread in a grating direction due to the parallelism of the beams, whereas the beams spread in the vertical direction.
Movement of the scale 2 can be detected by locating the scale 2 in a fashion traversing the direction of the spread beams and locating the photodetector 3 correspondingly.
The beams are also spread by providing such a modulated grating as has an inclination and a curvature gradually varying in a waveguide advancing direction.
To obtain the effects of the grating, the waveguide length must be at least 100 xcexcm so that a scale 15 is irradiated with beams having a small beam divergence and the angle xcfx86 meeting Equation (4).
Optical outputs from the laser light source are monitored by a similar laser structure which is provided at the other end of the waveguide.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.